1. Field of the Invention
The present invention relates to a MEMS (Micro Electro Mechanical System) variable optical attenuator, and more particularly to a MEMS variable optical attenuator with an improved fine beam shutter for controlling the amount of an optical signal traveling between transmitting and receiving optical waveguides.
2. Description of the Related Art
An optical attenuator used in optical communication systems denotes an optical component, which comprises a pair of transmitting and receiving terminals, and serves to attenuate light inputted via the receiving terminal due to an optical loss and then to output the attenuated light via the transmitting terminal.
Individually, the level of optical reception and transmission varies according to the configuration of a system. For example, the level of optical reception and transmission is determined by the difference of transmission loss due to the length of transmission distance of an optical fiber, the number of connecting portions of optical fibers, the number and performance of optical components such as optical branches used in a transmission line. Accordingly, there is required an optical attenuator when light with an excessive amount is received by an optical receiver. Further, the optical attenuator can be widely applied in evaluation, adjustment, and correction of communication equipment and optical measuring equipment.
The optical attenuators are divided into a fixed optical attenuator, in which the amount of attenuation of optical light is fixed, and a variable optical attenuator (VOA), in which the amount of attenuation of optical light is variable. There is required an optical attenuator with excellent reliability and small size at a reduced cost.
In order to satisfy the above requirements, the optical attenuators have been developed so as to have a MEMS structure using a thin film technique. In the MEMS variable optical attenuator, an actuator with a fine structure is formed on a substrate made of silicon or etc. using the thin film technique. Generally, the actuator is driven using a thermal expansion force or an electrostatic force, thus causing an electric potential difference to a beam shutter. Thereby, the amount of light transmitted from a transmitting terminal (also, referred to as an xe2x80x9cexit terminalxe2x80x9d) to a receiving terminal (also, referred to as an xe2x80x9centrance terminalxe2x80x9d) is controlled.
FIG. 1 is a schematic perspective view of a conventional MEMS variable optical attenuator using an electrostatic actuator. The conventional MEMS variable optical attenuator of FIG. 1 comprises a substrate 11 provided with a transmitting terminal 20 and a receiving terminal 30, an electrostatic actuator including driving electrodes 12a and 12b, a ground electrode 14, a spring 15 and a mobile mass portion 16, and a beam shutter 17 connected to the mobile mass portion 16 of the electrostatic actuator.
The driving electrodes 12a and 12b and the ground electrode 14 are supported on the substrate 11 by an oxide layer 19 (also, referred to as an xe2x80x9canchorxe2x80x9d). The mobile mass portion 16 is connected to the ground electrode 14 by the spring 15, and has a comb-type structure suspended from the substrate 11. Portions 13a and 13b extended from the driving electrodes 12a and 12b have a comb-type structure interdigitated with the comb-type structure of the mobile mass portion 16.
In FIG. 1, a driving signal is applied to the optical attenuator so that an electric potential difference occurs between the driving electrodes 12a and 12b and the ground electrode 14. Then, an electrostatic force is generated at the interdigitated comb structure between the mobile mass portion 16 and the extended portions 13a and 13b, and the mobile mass portion 16 is moved to the extended portions 13a and 13b by the electrostatic force. As the mobile mass portion 16 moves, the beam shutter 17 is interposed between the transmitting terminal 20 and the receiving terminal 30, thus partially cutting off light incident on the receiving terminal 30.
The above-described MEMS variable optical attenuator requires the uniform amount of the attenuation of light at any usable wavelength, and the minimal variation of the attenuation of light due to disturbance such as variations of time, wavelength, polarization, and vibration.
However, the conventional variable optical attenuator has problems such as a great wavelength dependent loss (WDL) and a great polarization dependent loss (PDL).
FIGS. 2a and 2b are schematic views illustrating optical attenuation effect by a planar beam shutter of the conventional variable optical attenuator.
With reference to FIG. 2a, light outputted from the transmitting terminal 20 and inputted to the receiving terminal 30 is partially cut off by the planar beam shutter 27. Here, the beam shutter 27 is made of silicon the same as the conventional actuator.
A part (R) of light with a relatively large amount is reflected by the beam shutter 27 and prevented from being incident on the receiving terminal 30. However, since the beam shutter 27 is made of silicon with excellent optical transmission, a further part (T) of light is incident on the receiving terminal 30. Another part (S1) of light is scattered and then incident on the receiving terminal 30, and yet another part (S2) of light is back-reflected and re-incident on the transmitting terminal 20. In order to improve optical cut-off effect of the planar beam shutter 27 made of silicon, a beam shutter 37, as shown in FIG. 2b, coated with a metal with high reflectivity (not less than approximately 90%) such as Au, Ni, Cu, Al, and Pt.
FIG. 2b shows the beam shutter 37 coated with Au as a reflective metal. The beam shutter 37 provided with an Au coating layer 38 reflects the part (R) of light with a relatively large amount, and prevents the part (R) from being incident on the receiving terminal 30, like FIG. 2a. 
However, the beam shutter 37 provided with the Au coating layer 38 reflects parts of light, thus generating the scattered parts (S1 and S2) of light. The scattered part (S1) of light is incident on the receiving terminal 30, and the scattered part (S2) of light is incident on the transmitting terminal 20. For example, when a beam shutter provided with an Au coating layer is used to cut off 50% of the total amount of light outputted from the transmitting terminal 20 to be inputted to the receiving terminal 30, the amount of the cut-off part (R) of light is approximately 49% of the total amount of light, and the amount of the scattered part (S1+S2) of light is approximately 1% of the total amount of light.
Although the amount of the scattered part of light is small, the amount of the back-reflected part of light is increased by the scattered part of light, and sensitively varied according to variations of wavelength and polarization.
Accordingly, when the scattered part of light is incident on the receiving terminal, the WDL and PDL of the variable optical attenuator are increased.
As described above, in the conventional MEMS variable optical attenuator, the amount of the back-reflected part of light is increased by the imperfect cut-off effect of the beam shutter, and the WDL and PDL are increased, thus reducing the reliability of the attenuator.
Accordingly, there has been required a MEMS variable optical attenuator, which minimizes the amounts of back-reflected and scattered parts of light to reach the transmitting terminal, and cuts off the transmitted and scattered parts of light so as not to reach the receiving terminal.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a MEMS variable optical attenuator, which minimizes the amount of back-scattered part of light due to the reflection of a beam shutter, and cuts off the transmitted and scattered parts of light so as not to reach the receiving terminal, thus reducing a wavelength dependent loss (WDL) and a polarization dependent loss (PDL).
In accordance with the present invention, the above and other objects can be accomplished by the provision of a MEMS variable optical attenuator comprising: a substrate having a flat upper surface; an electrostatic attenuator disposed on the upper surface of the substrate; transmitting and receiving terminals disposed on the substrate so that optical axes of the terminals coincide with each other; and a beam shutter moved to a designated position between the transmitting and receiving terminals by the actuator, wherein the beam shutter is provided with a first coating layer made of a material with a reflectivity of more than 90% and formed on a surface of the beam shutter, and a second coating layer made of a material with a reflectivity of less than 80% so that a part of light is transmitted by the second coating layer and with a photodisintegration rate of the transmitted light determined by a thickness of the second coating layer.
Preferably, the first coating layer may be made of one material selected from the group consisting of Au, Ni, Cu, Al, and Pt, and the second coating layer may be made of one material selected from the group consisting of Ti, TiO2, Cr, CrO2, W, Te, and Be. Further, preferably, the second coating layer may include: a first layer made of one material selected from the group consisting of Ti, Cr, W, Te, and Be; and a second layer made of one material selected from the group consisting of TiO2 and CrO2. 
Moreover, preferably, the beam shutter may have a planar structure tilted to the optical axes of the transmitting and receiving terminals. Otherwise, the beam shutter may include one plane being perpendicular to the optical axis of the receiving terminal and the other plane tilted to the optical axis of the transmitting terminal 20 at a designated angle less than 90xc2x0, i.e., an acute angle. In this case, the beam shutter may have a semi-wedge structure.
Preferably, the actuator may include: an electrode portion having a ground electrode and driving electrodes fixed to the substrate; a spring disposed on the substrate so that one end of the spring is connected to the ground electrode; and a mobile mass portion disposed on the substrate and connected to the other end of the spring so that the mobile mass portion is moved to the driving electrodes.
In this case, the first coating layer may be made of one material selected from the group consisting of Au, Ni, Cu, Al, and Pt, and the electrode portion may be coated with an electrode material the same as the material of the first coating layer so that a desired electrical conductivity is obtained. Otherwise, the second coating layer is made of one material selected from the group consisting of Ti, Cr, W, Te, and Be, and the electrode portion may be coated with an electrode material the same as the material of the second coating layer.
The MEMS variable optical attenuator is characterized in that it comprises the beam shutter provided with a first coating layer made of a material with a reflectivity of more than 90% and formed on a surface of the beam shutter, and a second coating layer made of a material with a reflectivity of less than 80% so that a part of light is transmitted by the second coating layer and with a photodisintegration rate of the transmitted light determined by a thickness of the second coating layer.
When a part of light is cut off by the MEMS variable optical attenuator, the second coating layer of the beam shutter reduces the amount of scattered light generated by the reflection and disintegrates a part of the transmitted light, and the first coating layer with the high reflectivity cuts off the amount of light transmitted by the second coating layer and scattered toward the receiving terminal. Accordingly, the MEMS variable optical attenuator reduces a reflection loss, a WDL, and a PDL.